A type of optical amplifier includes an EDFA (Erbium Doped Fiber optic Amplifier), which amplifies an input signal light to provide an output signal light having a constant power. The EDFA is excited with an excitation light generated by a laser diode, and the like. The input signal light is combined with the excitation light, and the combination light is supplied to the EDFA.
In such an optical amplifier, it is important to control an output power of the laser diode in order to keep the output signal light being constant. For controlling the laser diode, it is required to detect electric current flowing through it precisely. According to a conventional technology, a resistor is serially connected to a transistor, which drives the laser diode, to detect the electric current flowing through the laser diode. However, some voltage is lost at the resistor; and therefore, it is difficult to drive the laser diode with small quantity of electric current.